The present invention relates to a cable with at least one cable core having three or more conducting layers which are mutually separated by isolating layers, where said conducting layers include electrical conductors which are arranged helically with predetermined pitch length, and in particular to an electrical cable with reduced AC loss.
When using electrical power cables it is generally desirable to obtain as low power loss as possible. Cables adapted to have low losses are known from the prior art. The prior art discloses AC cables with at least one cable core having a number of conducting layers, which are mutually separated by isolating layers. The conducting layers are normally formed by electrical conductors, which are arranged helically with predetermined pitch angles.
The innermost conductor which may be of superconducting material is normally wound spirally around a central former and hereby forms a conducting layer. Likewise, the other conducting layers which may also be of superconducting material are normally wound spirally around the isolating layers adapted to separate the conducting layers. The number of conducting layers required depends on the desired use of the cable and on the current carrying capability of the tapes used.
The current distribution between the layers depends on the winding pitches, the layer radii, the layer thickness, and the resistivity of the layers of the conductors, as the inductance between the layers depends on the winding pitches. According to the prior art patches are therefore varied from layer to layer in a given way. As a result the current will distribute more equally between the conducting layers resulting in reduced AC losses.
WO 96/39705 discloses a cable with a central carrier body around which electrical conductors are arranged helically with predetermined pitch angles in at least three conductor positions. The pitch angles of the conductors in the individual conductor positions are selected such that they either increase or decrease in steps from position to position between a first value of the radial innermost conductor position and a second value of the radial outermost conductor position.
Even though the AC cables according to the prior art are found to be useful, they have the drawback that quite large variations of the winding pitches are required in order to obtain an equal current distribution. Furthermore, in practice, it is difficult to produce cables with these large variations in pitches.
Therefore the object of the invention is to provide an AC cable with low AC-loss and which overcome the disadvantages of the state of the art cables.
This object is achieved by a cable with at least one cable core having three or more conducting layers which are mutually separated by isolating layers, where said conducting layers include electrical conductors which are arranged helically with predetermined pitch length, in which said predefined pitch length increase in steps from layer to layer from the radial outermost conducting layer to an intermediate conducting layer located between the radial innermost conducting layer and the radial outermost conducting layer, and said predefined pitch length remain substantially constant or decrease in steps from layer to layer from said intermediate conducting layer to the radial innermost conducting layer.
Hereby, a homogenous distribution of the current in the cable and thus a reduction of the AC losses in the cable is obtained. Further, a cable having less variations of the winding pitches compared to cables of the prior art can be obtained. This is of interest as strongly varying pitches lead to differential thermal contraction of the different layers during cool-down of the cable. Further, small variation of the pitches between the layers is desirable as these result in beneficial mechanical properties of the cable.
The invention is based on the fact that the equations to be solved in order to design a cable of the above-mentioned type having reduced AC losses have been found to be very complex and cannot be solved analytically. The equations include a plurality of coupled parameters making the system difficult to solve, i.e. when varying one parameter, one or more other parameters may also be affected. When using a simplified model, the relation between some parameters can be explained e.g. the inductance is found to decrease as the radius of a conducting layer is increased, and the self-inductance of a conducting layer increases as the pitch angle increases. In cables not containing superconductors, layer resistance also plays a role. In practice it has been found that a homogenous current distribution can be obtained over the cross section of a cable of the above-mentioned type when designing the cable according to the invention, and hereby the above-mentioned advantages are obtained.
A preferred embodiment of the invention comprises the following features:
at least one of said isolating layers has a thickness different from the thickness of at least one of the other isolating layers,
at least one of said conducting layers has a thickness different from the thickness of at least one of the other conducting layers,
at least one of said isolating layers includes material which is different from the material of at least one of the other isolating layers, or
at least one of said conducting layers includes material, which is different from the material of at least one of the other conducting layers.
Hereby, a homogenous distribution of the current in the cable and thus a reduction of the AC losses in the cable is obtained. Even though a cable can be constructed having a single of the above-mentioned characteristics, two or more characteristics can be combined. Hereby, an increased degree of freedom when designing a cable of the above-mentioned type is obtained, as the desired homogenous distribution of the current in the cable can be obtained by varying one or more additional parameters, i.e. in addition to varying the winding pitches, the thickness of the isolating layers, the thickness of the conducting layers, the material of the isolating layers may be varied, and/or the material of the conducting layers may be varied. As a result, the variation in winding pitches can be reduced whereby further Improvement of the mechanical properties of the cable is obtained.
In a preferred embodiment at least one of said isolating layers is thicker than the radial outermost of the neighbouring isolating layers. In another preferred embodiment at least one of said conducting layers is thicker than the radial outermost of the neighbouring conducting layers.
In accordance with a further embodiment said thickness of said isolating layers and/or said conducting layers decrease in steps from layer to layer between a first value of the radial innermost isolating layer and a second value of the radial outermost isolating layer.
In a preferred embodiment, the resistivity of the conducting layer material increases in steps from layer to layer from a first value of the radial innermost conducting layer to a second value of the radial outermost conducting layer, i.e. the invention can be used in non-superconducting cables.
In another preferred embodiment that said conductors includes super-conducting material, i.e. the invention can be used in superconducting cables. For example, said super-conducting material is high-Tc super-conducting material.
The invention also relates to a method of producing a cable of the above-mentioned type.
The present invention further relates to the use of a cable according to the invention as a power cable.